A multi-feed detection sensor that detects multi-feed of sheets in a printer or the like uses piezoelectric elements.
The multi-feed detection sensor includes a pair of piezoelectric elements spaced from each other by a few centimeters with a sheet feed path interposed therebetween. One of the piezoelectric elements is configured for transmission, and the other piezoelectric element is configured for reception. A certain amount of installation space is required to provide the multi-feed detection sensor. In the past, therefore, the multi-feed detection sensor has been employed in a large- or medium-sized printer or scanner. In recent years, however, there has been an increasing demand for a reduction in size of the multi-feed detection sensor to allow the multi-feed detection sensor to also be mounted in a small-sized apparatus.
In an existing multi-feed detection sensor, a piezoelectric element in which area vibration occurs is bonded to, for example, an inner bottom surface of a cylindrical case with a bottom, to thereby configure a bimorph oscillator in which bending vibration occurs in the bottom surface of the case serving as a vibrating surface. The bending vibration occurs such that the center of the vibrating surface corresponds to the antinode of the vibration, and that a peripheral portion of the vibrating surface corresponds to the node of the vibration.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 10-224895